A combination of a rolling bearing provided outside a pump casing and a sliding bearing that is provided in a pumped water path (in a pump casing) and uses pumped water mixed with hard particles as a lubricant has been widely used as a bearing for journaling the main shaft of a horizontal drainage pump.
One of sliding bearings is a ceramic bearing having excellent wear resistance as disclosed in Japanese Unexamined Utility Model Publication No. S62(1987)-194919 (Patent document 1). This ceramic bearing employs an elastic support structure from a viewpoint of avoiding damage caused by local contact. This elastic support structure is constructed of a metal shell having a ceramic bearing shrink-fitted thereon and rubber mounted on the outer peripheral portion of the metal shell. In order to hold the axis of the ceramic bearing and the axis of a rolling bearing provided outside a pump casing horizontally, an elastic support structure of high rigidity is employed. In the beginning after assembling, hard rubber is hardly deformed to hold the axes horizontally and hence avoids local contact and provides stable sliding characteristics.
Further, one of bearings for vertical drainage pumps is a ceramic bearing as disclosed in Japanese Patent Laid-Open No. H6(1994)-147228 (Patent document 2). In this ceramic bearing, a spherical pivot is formed on the outer peripheral side of a metal case so as to avoid the local contact of a ceramic pad bearing fixed to the metal case and a metal ring is arranged on the outer peripheral side of the pivot and hard rubber is mounted on the metal ring in such a way as to surround the metal ring. This construction can prevent the ceramic pad bearing from being damaged by the local contact.
Still further, one of horizontal drainage pumps, as disclosed in Japanese Patent Laid-Open No. H6(1994)-346887 (Patent document 3), uses a sliding bearing of an oil lubrication type using a white metal as a bearing. This sliding bearing does not employ an elastic support structure, which is different from the ceramic bearing, and hence can hold an axis horizontally even if it is operated for a long period of time and can avoid damage caused by local contact.
On the other hand, one of sliding bearings having a sliding surface including a hard member and a soft member used for a diesel engine, a turbine, and the like is disclosed in Japanese Patent Laid-Open No. H10(1988)-252758 (Patent document 4). According to this patent document 4, in a sliding bearing having a sliding surface including hard members and soft members which are alternately arranged, the hard members and the soft members are arranged on the slant in a sliding direction. This construction can surely pass foreign matters in a lubricating oil moving along the sliding direction over the soft members arranged on the slant in the sliding direction even if the hard members are enlarged in width so as to have a necessary loading capacity. Hence, this construction can surely embed these foreign matters in the soft members, thereby providing the sliding bearing with a sufficient loading capacity and enhancing the ability of making the foreign matters in the lubricating oil embedded in the soft members to prevent the sliding bearing from being burnt.
Furthermore, one of sliding bearings used as bearings for an internal combustion engine such as automobile, ship, agriculture machine, and construction machine is disclosed in Japanese Patent Laid-Open No. 2002-147459 (Patent document 5). This sliding bearing includes a backing metal layer, a bearing alloy layer provided over the backing metal layer, and an overlay layer that is laminated over the bearing alloy layer and becomes a surface layer, wherein fine crater-shaped depressed portions are formed on the overlay surface and hard particles are sprayed onto the surface of the overlay layer. These depressions provide the surface with an oil storing function to increase the thickness of an oil film produced during operation to produce stable sliding characteristics.
However, the patent documents 1, 2 disclose that the local contact of the ceramic bearing is prevented but do not disclose that the bearing is damaged by hard particles mixed in pumped water used as a lubricant. In the bearings disclosed in the patent documents 1, 2, in a case where hard particles are mixed in the pumped water used as a lubricant, there is a possibility that the bearing part might be damaged by the hard particles.
Further, in the bearings disclosed in the patent documents 3 to 5, oil-lubricated bearing is used, which is different from the sliding bearing using pumped water as a lubricant for the bearing. This type of bearing has a problem that, in order to surely prevent oil from flowing outside the bearing, a seal structure of high reliability is indispensably required to increase the cost of a bearing unit. Still further, from a viewpoint of keeping a stable bearing function, oil to be used is required to have deterioration conditions checked on a regular basis. Hence, it cannot be said that this type of bearing gives sufficient consideration to maintenance-free performance.
The present invention has been made in view of these circumstances. The object of the invention is to provide a drainage pump and an underwater bearing unit that can enhance wear resistance to hard particles mixed in pumped water and prevent abrasive wear while providing maintenance-free performance by using the pumped water as a lubricant of a sliding bearing.